A polymerase is an enzyme that builds long chains of nucleic acids within living cells. These proteins synthesize DNA and RNA strands by adding individual building blocks, nucleotides, one by one. The precise work of polymerases underpins all genetic processes, making them indispensable for life. Without these enzymes, cells would be unable to copy their genetic material or translate it into functional proteins.
DNA Polymerase The Master Replicator and Repairer
DNA polymerase enzymes replicate the cell’s genetic blueprint, DNA, ensuring each new cell receives a complete copy. During cell division, these enzymes attach to a single strand of DNA, which serves as a template, and then add complementary nucleotides to form a new, matching strand. This precise process involves the formation of phosphodiester bonds between incoming nucleotides and the growing DNA chain.
Beyond replication, DNA polymerases also perform a proofreading function, correcting errors that may occur during nucleotide incorporation. For instance, in bacteria, DNA polymerase III is the primary replication enzyme, while DNA polymerase I removes RNA primers and fills gaps. Both can excise mismatched nucleotides. Eukaryotic cells utilize several types for replication, which also exhibit high fidelity. They are also involved in DNA repair pathways, filling in excised damaged segments and maintaining genome integrity.
RNA Polymerase The Gene Expresser
RNA polymerase is an enzyme that synthesizes RNA molecules from a DNA template in a process called transcription. This enzyme unwinds a segment of the DNA double helix and moves along one of the DNA strands, reading its nucleotide sequence. As it reads, RNA polymerase adds complementary RNA nucleotides, forming a new RNA strand that mirrors the genetic information encoded in the DNA. This newly synthesized RNA molecule grows in a 5′ to 3′ direction.
The RNA molecules produced by RNA polymerase include messenger RNA (mRNA), which carries genetic instructions for protein synthesis; transfer RNA (tRNA), which transports amino acids during protein assembly; and ribosomal RNA (rRNA), a structural component of ribosomes. This transcription process is the initial step in gene expression, converting information stored in DNA into functional molecules that dictate cellular activities. The regulation of RNA polymerase activity determines which genes are turned “on” or “off,” allowing cells to adapt and respond to their environment.
Other Vital Polymerases
Beyond DNA and RNA polymerases, other specialized polymerase enzymes perform unique functions. One such enzyme is reverse transcriptase, which synthesizes DNA using an RNA molecule as a template. Reverse transcription is a hallmark of retroviruses like HIV, allowing them to integrate their RNA-based genetic material into host cell DNA. It also finds application in molecular biology for generating complementary DNA (cDNA) from mRNA, a technique used in gene cloning and expression studies.
Another specialized polymerase is telomerase, a ribonucleoprotein complex that acts as a reverse transcriptase to maintain the ends of eukaryotic chromosomes, known as telomeres. Telomeres consist of repetitive DNA sequences that protect the genetic information on chromosomes from degradation and fusion during DNA replication. Telomerase adds these repetitive sequences to the telomere ends, counteracting the natural shortening that occurs with each round of cell division. Telomerase activity is often linked to cellular immortality in cancer cells, allowing them to divide indefinitely. Its decline in normal somatic cells is associated with cellular aging.